1. Field of the Invention
The present invention relates to a charge and discharge control circuit for controlling charge and discharge of a secondary battery, and to a rechargeable power supply device having the charge and discharge control circuit built therein.
2. Description of the Related Art
In recent years, as electronic equipment is downsized and becomes portable, a secondary battery is increasingly used. A lithium secondary battery is the most widely used secondary battery. Many problems with the lithium secondary battery are pointed out, including breakage, deterioration, and the shorter life of the battery due to overcharge and overdischarge. Among others, an overcharged lithium secondary battery which is charged beyond a safe battery voltage range may overheat and explode, and thus, a charge and discharge control circuit for protecting the lithium secondary battery is required to be highly reliable. Further, because the charge and discharge control circuit is mainly built in a battery for portable equipment, downsizing of the circuit is obviously indispensable.
The charge and discharge control circuit detects an overcharged state, an overdischarged state, an overcurrent state, or the like and controls charge and discharge currents. Generally, in order to avoid malfunction under the influence of noise or the like, when those states are detected, a charge and discharge control circuit confirms the detection after a certain delay time corresponding to each state. Generally, the delay time for overcharge detection is set to be several hundred milliseconds to several seconds.
However, when a charge current is intermittently supplied in a pulse-like manner, overcharge cannot be accurately detected in some cases. FIG. 4 is a graph of voltage across the secondary battery when the charge current is supplied in the pulse-like manner. The charge current is supplied during a time period tH while the charge current is not supplied during a time period tL. tCU is a delay time in detecting overcharge. During the time period tH, because the current flows through an internal impedance of the secondary battery, the voltage across the secondary battery is higher than a charge inhibition voltage. Meanwhile, during the time period tL, because the current does not flow, the voltage across the secondary battery is lower than the charge inhibition voltage. Here, if tCU>tH, then the voltage across the secondary battery goes down below the charge inhibition voltage during the delay time, and thus, an overcharge detection signal is reset. Because the overcharge is not detected, the charge and discharge control circuit cannot inhibit the charge.
In order to solve the problem, a method of controlling a charge and discharge control circuit as illustrated in FIG. 5 has been proposed (see Japanese Patent Application Laid-open No. 2003-257502). In the method of controlling a charge and discharge control circuit illustrated in FIG. 5, a neglect time which is longer than the time period tL is provided in the delay time tCU. This makes it possible for the charge and discharge control circuit to inhibit the charge even if the voltage across the secondary battery temporarily goes down below the charge inhibition voltage during the delay time tCU insofar as the time period during which the voltage is below the charge inhibition voltage is the time period tL illustrated in FIG. 5.
However, the method of controlling a charge and discharge control circuit as illustrated in FIG. 5 has the following problem. When both charge in a pulse-like manner and discharge in a pulse-like manner are carried out simultaneously, the charge is inhibited once, but, soon after that, the charge inhibition is canceled. This phenomenon is illustrated in FIG. 6. After the charge is inhibited because of the overcharge detection, when pulsed discharge makes the voltage across the secondary battery go down below a charge inhibition cancellation voltage, the charge inhibition state is canceled. When the charge inhibition state is canceled, a charger again starts the charge, and the voltage across the battery is raised again during the delay time tCU. One way to avoid this problem is thought to be lowering the charge inhibition cancellation voltage.
FIG. 7 is a block diagram of a typical rechargeable power supply device. A rechargeable power supply device 1 includes a secondary battery 2, a charge and discharge control circuit 100, and a charge and discharge current control circuit 10. In the charge inhibition state, a transistor 12 for charge control is off, and thus, a discharge current flows via a parasitic diode 14 of the transistor 12 for charge control. When the amount of the discharge current is large, an excessively heavy load is applied to the parasitic diode 14, which may result in deterioration or breakage. Accordingly, the method of avoiding cancellation of the charge inhibition due to pulsed discharge by lowering the charge inhibition cancellation voltage is not preferred from the viewpoint of securing the safety of the rechargeable power supply device.